Energy and power transfer has generally relied on three-phase generators. A rotating generator converts mechanical, chemical, and/or thermal energy into electrical power. A three-phase generator along with a transformer can provide a high-voltage electrical current for transmission. A transformer near the point of use can step the voltage back down to lower, single-phase voltage, which is typically required for residences and smaller business. Single-phase AC power of various frequencies is also sometimes produced on large vehicles, aircraft, and spacecraft for use by on-board devices.
Some machines require three-phase input power. Where three-phase power is not available, a single-phase to three-phase adapter will allow three-phase hardware to be used with a single-phase outlet. Typical single-phase to three-phase adapters use solid-state switching devices. In most environments, circuitry must be included to deal with switching noise generated by such adapters. These adapters must be replaced or repaired regularly, especially in harsh environments where the solid-state devices are prone to failure or otherwise have a limited useful life.